The few media reports who have done a little research claim that he's wielding a "batleth" (Klingon for "sword of honour"). Here's a picture of a batleth:

The batleth is a four-pronged, two-handed sword. It's wielded in such a way that all four prongs are facing forward, like this:

If you look back at the surveillance footage, you'll see that kid is brandishing a two-bladed dagger, that kinda looks like a batleth to the untrained eye, but the mere fact that he's holding it the wrong way round should tip you off that it's not one.

What's he's holding there is this:

The Valdris. A two-bladed, fantasy-styled dagger, roughly 60cm long, made of stainless steel (so it's designed to be an ornament, not an actual weapon).

This kid probably is a nerd, and may well be a Star Trek fan, but this is no Klingon weapon. It's not even a Star Trek weapon. Just to set the record straight.

I suspect that the journalists in question recognised the vague shape of the dagger as being reminiscent of the batleth, and jumped at the opportunity to belittle the offender by labelling him as a "Star Trek nerd."

UPDATE: Turns out the clerks at the convenience stores identified the weapon as a batleth. They were both wrong.